Method for making an injection-molded product having a partly thin portion

ABSTRACT

An injection molding method for molding a product having a partly thin portion comprising the steps of: providing a molding die including a slide block disposed in one of male and female dies and having in cross section a dimension corresponding to a dimension of the thin portion of the product; defining a cavity between the male and female dies with maintaining the slide block in a retracted position; injecting a molten resin material into the cavity through a gate; and advancing the slide block to project into the cavity before the resin material is solidified, thereby filling the cavity with the resin material while forming the thin portion.

The present application is a division of Ser. No. 07/289,744, filed Feb.8, 1989, which is a division of application Ser. No. 07/129,181, filedDec. 7, 1987.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention generally relates to an injection molding of asynthetic resin material and, more particularly, to an injection-moldedproduct having a partly thin portion and method and apparatus forinjection molding such a product.

2. Description of Prior Art

As is well known, one of the most suitable methods for mass productionof synthetic resin made products is the injection molding which iscarried out by injecting a molten resin into a cavity defined within amolding die. The injection molding permits a product having a relativelycomplicated shape to be manufactured easily because of less restrictionsin a design of the cavity. In the conventional injection molding,however, various problems have been experienced in the manufacture of aproduct having a partly thin portion.

For example, a vanity case need be provided with a concave portion foraccommodating cosmetics by reducing a wall thickness of the centralportion of a receptacle. In injection molding such a product, the cavityhas hitherto been defined as having two areas, one for the thickperipheral portion and the other for the thin central portion of theproduct, before the molten resin is injected into the cavity. This hasbeen considered satisfactory if the ratio between the thickness ofcentral portion and the thickness of peripheral portion is relativelysmall, that is, if the thickness of the former is not so small comparedwith the thickness of the latter. However, as the above ratio increases,that is, as the area of the cavity for the central portion becomesnarrower, resistance to the flow of resin material becomes larger. Thus,the resin material injected from a side gate, which opens into thecavity at a periphery thereof, tends to fill up the peripheral area ofthe cavity before flowing into the narrow central area. This results inan occurrence of a weld mark or line on the product at the opposite sideof the gate, which spoils an external appearance and reduces amechanical strength of the product. Furthermore, an extremely narrowcentral area of the cavity prevents the resin from filling it up becausethe flow of resin concentrates a pocket of air in the cavity to thecenter and increases the air pressure considerably. An attempt toincrease an injection pressure for overcoming the air resistance hasinvolved a burned on the product due to a burning of the compressed air.

The above problems might be solved by providing a pin point gate openinginto the cavity at a center thereof, instead of the side gate. In thiscase, however, a gate mark is necessarily formed on a center of theproduct and therefore an applicability of the pin point gate is limited.Another and more important disadvantage is that the peripheral area ofthe cavity tends to be insufficiently charged with the resin so that asink mark often occurs on the product. This is because the resin isfirst injected into the narrow area and then flows into the peripheralarea with a considerable loss of an injection pressure.

Therefore, the conventional injection molding does not permit the ratiobetween the thickness of the thin portion and of the thick portion to beconsiderably increased. For example, in a vanity case of a usual sizehaving a wall thickness of about 5 mm at the peripheral portion, thebottom wall defining the concave portion has been formed to have athickness of about more than 2 mm, i.e. more than 40% of the peripheralportion. Thus, the entire case has been bulky and not satisfactory.

Accordingly, an object of the present invention is to provide a methodand an apparatus for injection molding a product having a partly thinportion, which enable to reduce a thickness of the thin portion to alarge extent while maintaining a sufficient thickness of the remainingportion.

Another object of the invention is to provide an injection moldingmethod and apparatus which permits an efficient manufacture of a producthaving a partly thin portion with a good external appearance andstrength.

A further object of the invention is to provide an injection-moldedproduct in which a thickness of a thin portion can be as small asdesired.

SUMMARY OF THE INVENTION

According to the invention, an injection molding method for molding aproduct having a partly thin portion starts with the step of providing amolding die including a male die, a female die and a slide blockdisposed in one of the male and female dies, the slide block having incross section a dimension corresponding to a dimension of the thinportion of the product. A cavity is then defined between the male andfemale dies with maintaining the slide block in a retracted position.After a molten resin material is injected into the cavity through agate, the slide block is advanced to project into the cavity before theresin material is solidified, thereby filling the cavity with the resinmaterial while forming the thin portion.

During the advance of the slide block, the resin material is pushed awaytoward an area of the cavity surrounding the slide block andconsequently fills the cavity. A narrow area of the cavity,corresponding to the thin portion of the product, is defined by theadvance of the slide block after the resin material is injected into thecavity. Therefore, the resin material can flow into a whole area of thecavity without substantial increase of resistance to the flow andsubstantial loss of an injection pressure.

Preferably, the resin material is injected into the cavity with anamount larger than a required amount for molding the product. The excessof the resin material may be discharged from the cavity during theadvance of the slide block.

An apparatus according to the invention for injection molding a producthaving a partly thin portion comprises a molding die including a maledie and a female die for defining a cavity therebetween, the cavityhaving a volume larger than a volume of the product. The molding diealso includes a gate for injecting a molten resin material from aninjection molding machine into the cavity. A slide block is disposed inone of the male and female dies and having in cross section a dimensioncorresponding to a dimension of the thin portion of the product. Theslide block is movable between a retracted position whereat the volumeof the cavity is substantially maintained and an advanced position toproject into the cavity thereby reducing the volume thereof for formingthe thin portion. A drive means is provided for moving the slide blockand is adapted to advance the slide block before the resin materialinjected into the cavity is solidified.

Preferably, the molding die further includes a hollow spacecommunicating with the cavity for permitting an excess of the resinmaterial to be discharged from the cavity into the hollow space. In oneembodiment of the invention, the hollow space is normally closed by aclosure member fitted in the hollow space. Means for retracting theclosure member is provided to open the hollow space and is synchronizedwith the drive means.

According to another aspect of the invention, an injection-moldedproduct is provided which comprises a relatively thick-wall portion anda thin-wall portion formed unitarily and integrally with the relativelythick-wall portion, the thin-wall portion having a wall thickness ofless than 1 mm.

Other objects, features and advantages of the invention will becomeapparent from the following detailed description of the invention whentaken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view schematically illustrating a moldingapparatus according to an embodiment of the invention;

FIG. 2 is a view similar to FIG. 1 with a slide block in an advancedposition;

FIG. 3 is a partly sectioned perspective view illustrating an example ofan injection-molded product according to the invention;

FIG. 4 is a sectional view showing a molding apparatus according toanother embodiment of the invention;

FIG. 5 is a view similar to FIG. 4 with a slide block in an advancedposition;

FlG. 6 is a sectional view of a molding apparatus according to stillanother embodiment of the invention;

FIG. 7 is a view showing the same apparatus as in FIG. 6 with a slideblock being advanced and a closure member being retracted;

FIG. 8 is a sectional view of a molding apparatus according to furtherembodiment of the invention;

FIG. 9 is a view similar to FIG. 8 with a slide block in an advancedposition and a closure rod in a retracted position;

FIG. 10 is a fragmentary sectional view showing a hollow space and aclosure rod of a modified form;

FIG. 11 is a sectional view of a molding apparatus according to yetanother embodiment of the invention; and

FIG. 12 is also a sectional view showing still another embodiment of theinvention.

DETAILED DESCRIPTION OF THE INVENTION

Referring first to FIG. 1 of the drawings, a molding die 10 of anapparatus according to a first embodiment of the invention includes astationary female die 12 and a movable male die 14 which definetherebetween a cavity 16 of a shape corresponding to a configuration ofa product to be molded as shown, for example, in FIG. 3. Formed throughthe female die 12 is a sprue 18 which is connected at one end to anozzle of an injection molding machine (not shown) of any conventionaltype. The sprue 18 terminates at a small-dimensioned pin point gate 20opening in the cavity 16 at its substantially center portion. Thesurface of the male die 14 defining the cavity 16 is partly recessed toform a hollow space 22 in which a slide block 24 is fitted in afluid-tight manner. The slide block 24 has a cross-sectional shapecorresponding to a thin portion of the product and is advanced orretracted by a drive means 26 connected to the end of the slide blockopposite to the cavity 16. In this embodiment, the drive means 26comprises a shaft 28 fixed to the slide block 24 and a hydrauliccylinder 30 which moves the shaft 28 to slide the block 24 in the hollowspace 22. A hydraulic cylinder that is usually equipped in the injectionmolding machine for ejecting the molded product, may be used for thedrive means 26 if so desired.

Formed adjacent to the periphery of cavity 16 is a chamber 32 which iscommunicated with the cavity 16 via a small passage 34. As describedbelow, this chamber 34 is intended to permit an excessive amount ofmolten resin to flow thereinto when the slide block 24 is advanced.Therefore, the chamber 32 can be omitted in case that the injectionmolding machine enables to supply the molten resin into the cavity 16with a precise amount required for molding the product.

A molding operation using the above apparatus will now be described.First, the dies 10 are closed and the slide block 24 is set to itsretracted position as shown in FIG. 1. A molten resin 36 is theninjected into the cavity 16 from the nozzle and through the sprue 18 andthe pin point gate 20. The amount of resin to be injected in each cycleis predetermined to enable the molding of the product. When the slideblock 24 is in its retracted position, the cavity 16 has a volume largerthan the volume of the product and therefore the resin 36 does not fillup the cavity 16, leaving the marginal portion of the cavity 16uncharged. In many cases it is difficult to carry out the injection insuch a manner that the amount of injected resin precisely corresponds tothe amount required for molding the product. Thus, it is preferable tosupply the resin 36 with an amount larger than the calculated value sothat the cavity 16, when the slide block 24 is advanced, can beprevented from incomplete charging.

Thereafter, before the resin 36 is solidified, the hydraulic cylinder 30is actuated to advance the slide block 24 into the cavity 16 whereby theresin 36 existing in front of the block 24 is pushed away toward themarginal portion of the cavity 16. The slide block 24 continues toadvance until the end surface thereof defines a narrow gap of apredetermined width in cooperation with the bottom of the cavity 16,that is, until the cavity 16 corresponds to the configuration of productto be molded. When the cavity block 24 reaches such a position, thecavity 16 is filled up with the resin 36 as illustrated in FIG. 2. Thedies 10, after the resin is solidified, are released to eject the moldedproduct as in the conventional manner.

It is necessary that the slide block 24 be advanced before the resin 36is solidified. For example, if acrylonitrile-styrene resin is used forthe molding with temperature of the dies 10 being maintained at about60° C., a suitable timing for the advance motion of the slide block 24is about 0.5 second after the injection of the resin. Also, in case thatthe excessive amount of resin 36 is injected into the cavity 16, theadvance of the slide block 24 increases an internal pressure of thecavity 16, resulting in the excess of resin to flow out from the cavity16 into the chamber 32 through the passage 34. Thus, an over packing ofthe resin 36 in the cavity 16 can be prevented.

FIG. 3 illustrates an example of the product thus molded. This product40 comprises a thick-walled marginal portion 42 which has been molded inthe marginal portion of the cavity 16, and a thin-walled central portion44 which has been formed through the advance of slide block 24 and whichprovides a recess 46 for accommodating cosmetic material for example.The marginal portion 42 and central portion 44 are integrally andunitarily formed together, and a thickness t1 of the central portion 44is less than one-third (1/3) of a thickness t2 of the marginal portion42. In the present invention, the thickness t1 can be less than 1 mm.For instance, if the product 40 is designed as a part of a vanity case,the marginal portion 42 should have a thickness of at least about 5 mmin order to enable provisions of a latch tongue and/or a hinge. In thatcase, however, the central portion 42 can be formed with a thickness inthe range of 0.5 to 0.8 mm. As the result, size and weight of the entirevanity case may be reduced while maintaining a capacity for containingthe cosmetic material.

An apparatus illustrated in FIG. 4 is different from the aboveembodiment, mainly in a gate structure and a drive means for a slideblock. Thus, the apparatus also includes a molding die 50 comprising afemale die 52 and a male die 54 for defining a cavity 56 therebetween. Agate is in the form of a side gate 58 opening in the cavity 56 at theperipheral portion thereof and is connected to the nozzle of injectionmolding machine (not shown) through a runner 60 and a sprue 62 formed inthe female die 52. A chamber 64 is provided adjacent to the runner 60opposite to the gate 58 and is communicated with the runner 60 via apassage 66 which is dimensioned smaller than the gate 58. Fitted in ahollow space 68 in the male die 54 is a slide block 70 having a flange72 at the end opposite to the cavity 56. One end of a pair of springs74--74 abut against the flange 72 to normally urge the slide block 70 inits retracted position, i.e. toward the left-hand side in the figure. Arod 76 is slidably inserted in a guide hole 78 formed in the male die 54and extending perpendicularly to the hollow space 68. The rod 76 ismoved within the guide hole 78 by a hydraulic cylinder 80 and has aslant surface 82 which is in contact with a beveled end 84 of the slideblock 70 in such a manner that a downward movement of the rod 76 causesthe slide block 70 to advance against the elastic force of the springs74.

In a molding operation using the above apparatus, the injected moltenresin first flows into the runner 60 from the sprue 62 and then into thecavity 56 via the gate 58 and the chamber 64 via the passage 66. As thegate 58 is larger than the passage 66 in dimension and provides lessresistance to the flow of resin, most of the resin flows into the cavity56 while only a part thereof into the chamber 64. This must be takeninto consideration when an operator determines the amount of resin to beinjected, so that the cavity 56 may be supplied with the resin of anamount corresponding to or slightly larger than the amount required formolding the product. Thereafter, the rod 76 is lowered by the cylinder80 to advance the slide block 70 as shown in FIG. 5, whereby the cavity56 is filled up with the resin to mold the product having thepredetermined thin portion as described hereinabove. In case that theamount of resin injected into the cavity 56 exceeds the reduced volumethereof in FIG. 5, the excess of the resin flows back to the runner 60through the gate 58 and further flows into the chamber 64 via thepassage 66. As in the above embodiment, if the injection molding machineallows the cavity 56 to be supplied with the precise amount of resin,the chamber 64 can be omitted.

FIG. 6 illustrates an apparatus according to another embodiment of theinvention, this apparatus being similar to that of FIG. 4 and thereforethe same or corresponding parts being indicated by the same referencenumerals. In this embodiment, the male die 54 is drilled to provide abore 86 extending up to the runner 60 and a closure rod 88 is slidablyand fluid-tightly fitted in the bore 86. The bore 86 has at the endopposite to the runner 60 an enlarged portion 90 for accommodating aflange 92 of the closure rod 88 and a spring 94 which urges the rod 88toward the left-hand side in the figure. A piston 96 of a hydrauliccylinder 98 abuts against the flange 92 to normally maintain the closurerod 88 in its advanced position where the bore 86 is completely closedby the rod 88, with the front end surface of the latter being flush withthe bottom of the male die 54 defining the runner 60.

For carrying out a molding operation, the dies 50 are closed and theslide block 70 as well as the closure rod 88 is first set to theposition shown in FIG. 6. Then a molten resin is injected into thecavity 56 through the sprue 62, runner 60 and the gate 58. The injectionmolding machine (not shown) should be so arranged that the amount ofresin to be supplied into the cavity 56 exceeds the amount necessary forthe molding of product. However, in order to avoid the over packing ofthe cavity 56 by the advance of the slide block 70 as described below,the cavity 56 preferably is not entirely filled with the resin. Adesirable rate of an uncharged area depends on a size of the slide block70 and a distance of advance thereof, and in the illustrated embodimentthe amount of the injected resin is so adjusted that about a quarter(1/4) of the initial volume of the cavity 56 is left uncharged.

Thereafter and before the injected resin is solidified, the slide block70 is advanced by the rod 76 as shown in FIG. 7 so that the cavity 56reduces the volume and becomes filled up with the resin to mold theproduct, as in the above embodiment. At the same time, the cylinder 98is actuated to retract the piston 96, which also causes the rod 88 toretract by the force of the spring 94. By this movement, the front endportion of the bore 86 is opened to define a space 100 whichcommunicates with the runner 60 and permits the resin to flow thereintofrom the runner 60. Therefore, as the volume of the cavity 56 reduces,excess of the resin can flow back into the runner 60 through the gate58, thus avoiding over packing of the cavity 56.

Preferably, the rod 88 is again advanced by the cylinder 98 as soon asthe slide block 70 reaches its advanced position. This increases thepressure in the resin which is transmitted to the cavity 56 via therunner 60 and the gate 58, whereby a shrinkage during the cooling can besupplemented to advantageously prevent occurrence of sink marks on theproduct.

In a modification illustrated in FIGS. 8 and 9, the slide block 70 andthe closure rod 88 are moved by the common drive means. That is, theguide hole 78 in the male die 54 is elongated to extend beyond the bore86 and the rod 76 includes an extension 102 having a second slantsurface 104 which is inclined in a direction opposite to that of thefirst slant surface 82. The flange 92 of the closure rod 88 also has abeveled end 106 complementary to and abutting against the second slantsurface 104. Thus, the downward movement of the rod 76 causes the slideblock 70 to advance against the springs 74 while permitting the closurerod 88 to retract by the force of the spring 94, as shown in FIG. 9.

FIG. 10 shows another arrangement for the resin receiving space, inwhich a bore 108 is formed as a continuation of the sprue 60 at theopposite side of the gate and is adapted to be closed or opened by aclosure rod 110.

An apparatus of FIG. 11 is similar to the apparatus of FIG. 4 in that ahollow space 112 is provided in the female die 52 and communicated withthe runner 60 via the passage 66 which is smaller in dimension than thegate 58. The space 112 has a volume substantially less than the volumeof the space 64 in FIG. 4. A bore 114 is drilled in the male die 54 toextend up to the space 112 and is normally closed by a closure rod 116which is secured in position by a spring 118 and a shaft 120 having aflange 122. The amount of resin to be injected is so determined that thecavity 56 may receive the resin of substantially larger amount thanrequired for the molding of product while the space 112 is almost filledwith the resin. When the volume of cavity 56 is reduced by the advanceof the slide block 70, excess of the resin flows back into the runner 60through the gate 58 and then into the space 112 via the passage 66. Theinternal pressure of the space 112 is thus increased, causing theclosure rod 116 to retract against the spring 118 with opening the bore114. One of the advantages of these arrangements is that the space 112and an opened portion of the bore 114 is positively filled with theresin, which promotes the cooling of the resin.

FIG. 12 illustrates a modified form of the apparatus of FIG. 1 havingthe pin point gate. Because the small dimension of the pin point gate 20permits the injected resin to quickly solidify at the gate, it isnecessary to provide an outlet for discharging the excess of resinseparately from the gate 20. In this apparatus, an inverted L-shapedpassage 124 is formed in an ejector pin 126 mounted in the male die 14to communicate the peripheral portion of the cavity 16 with a hollowspace 128 in the male die 14. One end of the passage 124 opening to thecavity 16 is located at the reverse side of the product to be molded,which may improve an external appearance of the product.

As it should be understood from the foregoing description, according tothe invention the thin portion is formed by the advance of the slideblock after the resin is injected into the cavity. This enables to moldthe product having thin portion of a desired thickness positively andefficiently without involving the problems that have been encountered inthe prior art technique. The injection-molded product according to theinvention includes the thin portion of less than 1 mm thickness,allowing reduction of size and weight of a final product.

Although the present invention has been described with reference to thepreferred embodiments thereof, many modifications and alterations may bemade within the spirit of the invention.

What is claimed is:
 1. A method for injection molding a product having a thick-wall portion and a thin-wall portion comprising the steps of:providing a molding die including a male die and a female die disposed with respect to each other so as to define a cavity between the male die and the female die, the cavity having a cross section corresponding to the thick-wall portion of the product, the molding die also including a slide block movably disposed in one of the male die and female die and a gate for permitting the injection of molten resin material into the cavity; injecting a molten resin material of an amount larger than that required for molding the product into the cavity through the gate while maintaining said slide block in a retracted position; advancing the slide block to project into the cavity before the resin material is solidified to form the thin-wall portion of the product between a face of the slide block and a confronting portion of the molding die; defining a hollow space within the molding die communicating with said cavity substantially simultaneously with the slide block advancing step to permit excess of the resin material to be discharged from the cavity into the hollow space; and reducing a volume of said hollow space immediately after the slide block completes the advancing step to thereby increase the pressure in the resin material.
 2. The method of claim 1 wherein the defining step comprises the step of retracting a closure member fitted within a bore coupled to the cavity to enlarge the volume of the bore.
 3. The method of claim 2 wherein the reducing step comprises the step of driving the closure member forward to reduce the volume of the bore.
 4. The method of claim 3 wherein the driving step comprises the step of actuating a drive means connected to the closure member. 